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Borehole temperature distribution during Thermal Response Test

Study on shallow geothermal energy in Republic of Croatia as part of EU funded project „Research and promotion of shallow geothermal potential in Croatia” included drilling of exploratory borehole on location of Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb. Grouted 130 m deep borehole incorporates double U pipe heat exchanger equipped with fiber optic cables for distributed temperature sensing. Separate optic cables are placed inside one U pipe of heat exchanger and in borehole grouting. Such configuration of measuring equipment enables monitoring of vertical temperature distribution along the borehole heat exchanger and horizontal temperature distribution inside the borehole cross section. Borehole heat exchanger was subjected to the distributed thermal response test consisting of three stages to determine thermal parameters relevant for the design of ground coupled heat pump system. First measurements are carried out without water circulation to determine undisturbed ground temperature profile, second stage involved 120 hour long heating of the underground with constant heat flux and in the last phase of the test, thermal recovery of the borehole was observed for two weeks. Analytical interpretation of thermal response test results yielded effective thermal conductivity of 1.66 W/(m K) and effective borehole thermal resistance of 0.072 (m K)/W. Vertical temperature profiles of undisturbed and disturbed ground are presented and influence of fluid temperature distribution on a results of thermal response test discussed.

Distributed thermal response test; shallow geothermal energy; ground thermal parameters

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